Refactor textsecure.protos -> textsecure.protobuf

DRY up protobuf declarations and move to a slightly briefer naming
convention.

Also dropped some ArrayBuffer -> string conversions as
ProtoBuf.js handles ArrayBuffers just fine, and in fact, more
efficiently than strings.

Finally, dropped the btoa() wrappers, because that incurs an extra
string -> string conversion before the protobuf's internal string ->
array buffer conversion. In lieu of btoa, we can simply pass in the
optional string encoding argument to the protobuf's decode method,
which in these cases should be 'binary'.

Related: #17

js/protobufs.js

@@ -0,0 +1,21 @@
+;(function() {
+
+    function loadProtoBufs(filename) {
+        return dcodeIO.ProtoBuf.loadProtoFile({root: 'protos', file: filename}).build('textsecure');
+    };
+
+    var pushMessages     = loadProtoBufs('IncomingPushMessageSignal.proto');
+    var protocolMessages = loadProtoBufs('WhisperTextProtocol.proto');
+    var deviceMessages   = loadProtoBufs('DeviceMessages.proto');
+
+    window.textsecure = window.textsecure || {};
+    window.textsecure.protobuf = {
+        IncomingPushMessageSignal : pushMessages.IncomingPushMessageSignal,
+        PushMessageContent        : pushMessages.PushMessageContent,
+        WhisperMessage            : protocolMessages.WhisperMessage,
+        PreKeyWhisperMessage      : protocolMessages.PreKeyWhisperMessage,
+        DeviceInit                : deviceMessages.DeviceInit,
+        IdentityKey               : deviceMessages.IdentityKey,
+        DeviceControl             : deviceMessages.DeviceControl
+    };
+})();